PC Processors Guide by x86.org

"The Intel 8086, a new microcomputer, extends the midrange 8080 family into the 16-bit arena. The chip has attributes of both 8- and 16-bit processors. By executing the full set of 8080A/8085 8-bit instructions plus a powerful new set of 16-bit instructions, it enables a system designer familiar with existing 8080 devices to boost performance by a factor of as much as 10 while using essentially the same 8080 software package and development tools.

"The goals of the 8086 architectural design were to extend existing 8080 features symmetrically, across the board, and to add processing capabilities not to be found in the 8080. The added features include 16-bit arithmetic, signed 8- and 16-bit arithmetic (including multiply and divide), efficient interruptible byte-string operations, and improved bit manipulation. Significantly, they also include mechanisms for such minicomputer-type operations as reentrant code, position-independent code, and dynamically relocatable programs. In addition, the processor may directly address up to 1 megabyte of memory and has been designed to support multiple-processor configurations."

-- Intel Corporation, February, 1979


In 1979, Intel introduced the 8086 and 8088 microprocessor extensions to the 8080 product line. Since that time, the x86 product line has gone through six generations and become the most successful microprocessor in history. Much of this success was due to the success of the IBM-PC and its clones. Therefore Intel was at the right place at the right time when IBM made their historic decision to use the 8088. Today, the x86 market is a multi-billion dollar industry, selling tens-of-millions of units per year.

The huge popularity of these x86 chips has lead to a prosperous x86 clone industry. AMD, Cyrix, IBM, TI, UMC, Siemens, NEC, Harris, and others have all dabbled in the x86 chip industry. Today, AMD, Cyrix, and Centaur are still actively competing. However, there are numerous stories that approximately one dozen other companies are actively working to create a x86 clone chip. Ultimately, the market will decide who survives and who perishes in this cutthroat chips business.


Merced is Intel's future-generation microprocessor architecture; Merced is not Intel's next-generation x86 microprocessor. Instead, Merced is a completely new microprocessor design and instruction set. Merced will have the means to run legacy x86 programs via a hardware translation mechanism.

The Merced design is a further departure from its x86 predecessors. Merced is not CISC (Complex Instruction Set Computer), RISC (Reduced Instruction Set Computer), but closely resembles a VLIW design (Very Long Instruction Word). Intel doesn't want to call this chip VLIW, ostensibly for political reasons (not invented here). Instead, Intel has coined the term EPIC -- Explicitely Parallel Instruction Computing. For all intents and purposes, EPIC is VLIW.

First versions of Merced will run between 600 MHz and 1000 MHz (1 GHz). x86 compatibility is acheived internally via a hardware translation mechanism. This translation mechanism enables Merced to run your existing Windows and DOS applications. Intel claims that Merced will never run x86 programs as fast as the current state-of-the-art x86 microprocessors. Therefore, Intel offers Merced into the server and workstation market, and will offer x86 compatibility as a matter of convenience.


The Xeon is a Pentium II on steroids. The Pentium II contains a P6 (Pentium Pro) core with a 1/2-speed 2nd-level cache (L2 cache). Xeon differs from Pentium II in that the L2 cache runs at full processor speed. Pentium II connects to the motherboard in a slot named Slot-1. Xeon is not slot-compatible with Pentium II. Instead, Xeon uses a new slot named Slot-2.

Xeon is Intel's high-end microprocessor brand for the computer server market. As such, Xeon is priced much higher than Pentium II or Celeron.


The Celeron microprocessor is a stripped down Pentium II. This chip is affectionately knows as "the Castrated One." The Celeron is offered without any 2nd-level cache, making its performance lackluster, and reportedly slower than a Pentium (with MMX) running at nearly one-half of its speed.

Intel was completely caught off-guard by the advent and popularity of the sub-$1000 PC. In fact, the sub-$1000 PC completely ruins Intel's marketing game plan. Therefore, Intel hastily created the Celeron in an attempt to regain some marketshare they lost to their competition.

Pentium II

The Pentium II is a glorified Pentium Pro with MMX extensions. The Pentium II has a different package than the Pentium Pro. This new package is called "Slot-1." Officially, Intel claimed technical reasons for needing Slot-1. Industry pundits claimed that slot-1 was devised to thwart industry competition with the Pentium Pro, and further the Intel monopoly. Strangely, the "technical reasons for needing slot-1" evaporated as soon as the Pentium Pro was dead; as Intel somehow made a "breakthrough" that they had previously claimed was impossible (thereby "needing" to invent the proprietary slot-1).

Slot-1 was also promised to be the upgrade path for consumers -- leading many years into the future. Unfortunately, every time Intel has made this promise, first with the 80486, then with the Pentium, the promise has been broken. As soon as Intel saturated the saturated the market with slot-1 computers, they announced the future high performance upgrade path would be "slot-2."

The Pentium II can address up to 64 GB of main memory, but has cache limitations preventing memory use above 512 MB.

The Pentium Pro

The Pentium Pro was introduced in November 1995 as Intel's 6th generation x86 design -- code-named the "P6." The Pentium Pro offered some minor programming enhancements, four more address lines, and a large 2nd-level cache. By this time, the entire "secret" programming features of the Pentium had been revealed (mostly at the Intel Secrets Web Site: http://www.x86.org). Therefore, Intel abandoned their attempts at keeping most of their new P6 programming features as a secret. By adding four more address lines, the Pentium Pro could address up to 64 GB of main memory. The addition of the 2nd-level cache gave the Pentium Pro a nice performance boost, but was very expensive to manufacture.

Intel continued their attempts at closing the architecture to the exclusion and elimination of their competition. Intel managed to gain patent protection for some pins on the Pentium Pro socket; thus making this chip very difficult to clone without substantial legal liability. However, Intel wasn't paranoid enough. Intel introduced the Pentium II under the guise that the Pentium Pro could never achieve their performance goals. Intel alleged that the Pentium Pro 2nd-level cache could never run faster than 200 MHz, and therefore they must discontinue the development of this product line. The Pentium II abandoned the socket approach to microprocessors, and introduced the "slot" concept. The slot-1 of the Pentium II was further enshrouded in patent protection, thereby further raising the bar to the cost of competition. Now that the Pentium Pro is dead (along with the possibility that their competition will clone this product), Intel has ironically announced a 400 MHz Pentium II with a full-speed 2nd-level cache.

Regardless of Intel's continued monopolistic business practices, the P6 product line has diversified and flourished. The Pentium II added MMX enhancements and a variety of 2nd-level cache options. Intel has created the Celeron brand to compete in the sub-$1000 market. The Xeon was introduced to compete in the server market with a 100 MHz system bus. As time goes on, Intel will continue to diversify the P6 family product line, most likely with a 200 MHz system bus.

Intel's competitors have gone in a different direction than Intel. Intel's competitors have stayed with a Pentium-compatible pin-out. AMD has continued to develop the K6 processor and added MMX enhancements. Cyrix has added MMX enhancements to the MII product line. Centaur products always contained MMX enhancements. These three companies combined their abilities to create a common set MMX-3D instruction set extensions. All three companies have announced plans to create a 100 MHz system bus and an integrated 2nd-level cache (though Intel-fellow Fred Pollack has said publicly that the 2nd-level cache integration is electronically impossible).

The Pentium

The Pentium processor was a big departure from all past Intel x86 processors. The Pentium name signaled the end to the 80x86 nomenclature, and was spurred by losing a trademark dispute against AMD. The Pentium processor contained more than one execution unit -- making it superscalar. Intel no longer needed (or wanted) any second-source fabs manufacturing their microprocessors; Intel wanted all of the profits to themselves. The Pentium also added many programming enhancements though Intel tried to keep all of them secret.

Intel rapidly diversified the Pentium product line. The original Pentium product ran at 60 and 66 MHz. Shortly thereafter, Intel introduced 90, 100, 120, 133 MHz versions of the popular processor. Intel introduced low-power versions of the Pentium to be used in notebook computer applications. Finally, Intel introduced the MMX-enhanced processors.

AMD and Cyrix didn't sit idly by and watch Intel expand and dominate the market. AMD introduced the K5 processor -- their first in-house x86 design. However, the K5 was late to market, and was very slow. In response to their bleak outlook for the K5, AMD bought Nexgen. Nexgen created their own x86-compatible microprocessor, calling it the Nx586. At the time of the acquisition, Nexgen had already finished the design of their next-generation processor core -- the Nx686. AMD used the Nx686 core and created the successful K6 processor. AMD has continued to upgrade this processor to include MMX, and other enhancements.

During this time Cyrix introduced the 6x86. The 6x86 was pin-compatible with the Pentium, though the 6x86 nomenclature might lead the consumer to believe that it is a 6th-generation (Pentium Pro) compatible chip. The 6x86 has also been enhanced with MMX instructions as the 6x86 MX. Cyrix has continued to enhance this chip in their attempt to gain more market share.

During the Pentium era, a new Intel competitor emerged. Centaur Technologies (a wholly owned subsidiary of IDT) created a fast, cheap, and somewhat low power Pentium compatible chip. Centaur has a low-end (low-cost) market focus. Some industry pundits have called this marketing strategy "bottom-feeding." However, with the emergence and overwhelming popularity of the sub-$1000 PC, Centaur may end up having the last laugh.

The 80486

The 80486 offered little in the way of architectural enhancements over its 80386 predecessor. The most significant enhancement of the 486 family was the integration of the 80387-math coprocessor into the 80486-core logic. Now, all software that requires the math coprocessor could run on the 80486 without any expensive hardware upgrades.

Like the 80386 SX, Intel decided to introduce the 80486 SX as a cost-reduced 80486 DX. Unfortunately, Intel chose to ensure that these processors were neither pin-compatible, nor 100% software compatible with each other. Unlike the 80386 SX, the 80486 SX enjoyed the full data bus and address bus of its DX counterpart. Instead, Intel removed the math coprocessor, thereby rendering the 80486 SX somewhat software incompatible with its DX counterpart. To further complicate matters, Intel introduced the 80487 SX -- the "math coprocessor" to the 80486 SX. Intel convinced vendors to include a new socket on the motherboard that could accommodate the 80486 SX and 80487 SX as an expensive hardware upgrade option. Unbeknownst to the consumer, the 80486 SX was an 80486 DX with a non-functional math unit (though later versions of the chip actually removed the math unit). The 80487 SX was a full 80486 DX with a couple of pins relocated on the package -- to prevent consumers from using the cheaper 80486 DX as an upgrade option. In this regard, Intel created a marketing deception. Intel marketed the 80487 SX as a math coprocessor to the 80486 SX. In reality, the 80487 SX electronically disabled the 80486 SX when installed, thereby relegating this chip to the status of an expensive space heater. Sadly, the consumer never knew or even suspected Intel of playing such manipulative games.

Also like the 80386, the Intel began to diversify their 80486 offerings. Low-power versions of the chip were introduced. The 80486 SL was introduced along with the 80386 SL as an integrated, low-power chip for notebook applications. The 80486 DX2 and DX4 were introduced, which doubled and tripled the core clock frequency. Power-saving features from the SL were introduced in later versions of the DX4. Finally, after Intel introduced the Pentium chip, they produced a version of the Pentium that was pin-compatible with the 80486. They called this chip an "overdrive" processor.

Likewise, AMD and Cyrix continued to pursue their own 486-compatible chip solutions. AMD introduced many Am486 variants. Cyrix continued their nomenclature of calling an 80486-compatible chip, the Cyrix 5x86. TI continued to manufacture Cyrix chips, and eventually started their own in-house microprocessor design (though the effort eventually failed). UMC entered the CPU market, but later withdrew because of patent infringement problems. IBM began manufacturing for Cyrix, and still pursuing their own microprocessor designs (the Blue Lightning series).

The 80386

In 1985, Intel introduced the 80386. Like the 80286 before it, the 80386 added significant programming and addressibility enhancements. Protected mode was enhanced to allow easy transitions between it and real mode (without resetting the microprocessor). Another new operating mode (v86 mode) was introduced to allow DOS programs to execute within a protected mode environment. Addressibility was further enhanced to 32-bits, giving the 80386 four gigabytes of memory addressibility (2^32 = 4 GB).

Also like the 80286, the 80386 was not introduced in any computer systems for many years after its introduction. Compaq was the first mainstream company to introduce an 80386-based computer -- beating IBM to market. Regardless, the 80386 enjoyed a very long life for home and business computer users. This long life was largely due to the programming extensions in the 80386 -- namely the ability to create a protected mode operating system to take advantage of all 4 GB of potential memory while still being able to run legacy DOS applications.

Shortly after the 80386 was introduced, Intel introduced the 80386 SX. To avoid confusion, Intel renamed the 80386 to the 80386 DX. The SX was a cost-reduced 80386 with a 16-bit data bus, and 24-bit address bus. The 16-bit data bus meant the SX was destined to have lower memory throughput than its DX counterpart; while the 24-bit address bus mean that the SX could only address 16 MB of physical memory. Regardless of the address bus and data bus differences, the SX and DX were software compatible with each other. Intel also introduced the 80376 as part of the 80386 family. The 376 was an 80386 SX that exclusively ran in protected mode.

During its long reign, the 80386-based computer began to evolve. Chipset vendors began dreaming of ways they could help improve the performance of the computer, thus giving their products a competitive advantage. One of the innovations was the introduction of the memory cache. The memory cache within the chipset would play a huge role in Intel's future product plans. First, Intel introduced a cache . Later, they incorporated the cache into the microprocessor itself. Intel also made their second failed attempt at chip integration. The 80386 SL integrated core logic, chipset functionality, and power-saving features into the microprocessor.

During this time, the popularity of the personal computer, and most notably their Intel microprocessors, didn't escape the notice of many entrepreneurs wishing to cash in on Intel's business. AMD began their own "x86" microprocessor division. IIT began cloning the Intel math coprocessors. Other small startups, such as Cyrix and Nexgen, decided they too could design an Intel-compatible microprocessor. The aspirations of these companies didn't bode well within Intel. Shortly thereafter, Intel began taking measures to ensure their own dominance in the industry -- to the exclusion of everybody else. Hence, Intel began what many believe are anti-competitive (illegal) business practices.

In spite of Intel's business practices, many 80386 clones began to appear. AMD marketed the Am386 microprocessors in speeds from 16 MHz to 40 MHz, though it was possible to overclock this chip up to 80 MHz. IBM introduced the 386 SLC, which featured a low-power 386 with an integrated 8-KB cache. IBM created other 386/486 hybrid chips -- some that were pin-compatible with Intel, and others that were not. Chips and Technologies created their own 386 clone. Cyrix stunned everybody by offering a 386 pin-compatible CPU, but called it a 486 (a nomenclature pattern that Cyrix still uses). Texas Instruments served as a foundry for Cyrix, and negotiated rights to produce chips under their own name. Eventually, TI produced their own chips (based on the Cyrix core), with their own unique enhancements.

The 80286

In 1982, Intel introduced the 80286. For the first time, Intel did not simultaneously introduce an 8-bit bus version of this processor (ala 80288). The 80286 introduced some significant microprocessor extensions. Intel continued to extend the instruction set; but more significantly, Intel added four more address lines and a new operating mode called "protected mode." Recall that the number of address lines directly relates to amount of physical that can be addressed by the microprocessor. The 8086, 8088, 80186, and 80188 all contained 20 address lines, giving these processors one megabyte of addressibility (2^20 = 1MB). The 80286, with its 24 address lines, gives 16 megabytes of addressibility (2^24 = 16 MB).

For the most part, the new instructions of the 80286 were introduced to support the new protected mode. Real mode was still limited to the one megabyte program addressing of the 8086, et al. For all intents and purposes, a program could not take advantage of the 16-megabyte address space without using protected mode. Unfortunately, protected mode could not run real-mode (DOS) programs. These limitations thwarted attempts to adopt the 80286 programming extensions for mainstream consumer use.

IBM was spurred by the huge success of the IBM PC and decided to use the 80286 in their next generation computer, the IBM PC-AT. However, the PC-AT was not introduced until 1985 -- three years after introduction of the 80286.

During the reign of the 80286, the first "chipsets" were introduced. The computer chipset was nothing more than a set of chips that replaced dozens of other peripheral chips, while maintaining identical functionality. Chips and Technologies became one of the first popular chipset companies.

Like the IBM PC, the PC-AT was hugely successful for home and business use. Intel continued to second-source the chips to ensure an adequate supply of chips to the computer industry. Intel, AMD, IBM, and Harris were known to produce 80286 chips as OEM products; while Siemens, Fujitsu, and Kruger either cloned it, or were also second-sources. Between these various manufacturers, the 80286 was offered in speeds ranging from 6 MHz to 25 MHz.

The 80186 / 80188

Intel continued the evolution of the 8086 and 8088 by introducing the 80186 and 80188. These processors featured new instructions, new fault tolerance protection, and was Intel's first of many failed attempts at the x86 chip integration game.

The new instructions and fault tolerance additions were logical evolutions of the 8086 and 8088. Intel added instructions that made programming much more convenient for low-level (assembly language) programmers. Intel also added some fault tolerance protection. The original 8086 and 8088 would hang when they encountered an invalid computer instruction. The 80186 and 80188 added the ability to trap this condition and attempt a recovery method.

Intel integrated this processor with many of the peripheral chips already employed in the IBM-PC. The 80186 / 80188 integrated interrupt controllers, interval timers, DMA controllers, clock generators, and other core support logic. In many ways, this chip was produced a decade ahead of its time. Unfortunately, this chip didn't catch on with many hardware manufacturers; thus spelled the end of Intel's first attempt at CPU integration. However, this chip has enjoyed a tremendous success in the world of embedded processors. If you look on your high performance disk driver or disk controller, you might still see an 80186 being used.

Eventually, many embedded processor vendors began manufacturing these chips as a second source to Intel, or in clones of their own. Between the various vendors, the 80186/80188 was available in speeds ranging from 6 MHz to 40 MHz.

The 8086 / 8088

The 8086 and 8088 were binary compatible with each other, but not pin-compatible. Binary compatibility means that either microprocessor could execute the same programs. Pin-incompatibility means that you can’t plug the 8086 into the 8088 and visa versa, and expect the chips to work. The new "x86" chips implemented a Complex Instruction Set Computer (CISC) design methodology.

The 8086 and 8088 both feature twenty address pins. The number of address pins determines how much memory a microprocessor can access. Twenty address pins gave these microprocessors a total address space of one megabyte (2^20 = one megabyte).

The 8086 and 8088 featured different data bus sizes. The data bus size determines how many bytes of data the microprocessor can read in each cycle. The 8086 featured a 16-bit data bus, while the 8088 featured an 8-bit data bus. IBM chose to implement the 8088 in the IBM-PC, thus saving some cost and design complexity.

At the time IBM introduced the IBM-PC, a fledgling Intel Corporation struggled to supply enough chips to feed the hungry assembly lines of the expanding personal computer industry. Therefore to ensure sufficient supply to the personal computer industry, Intel subcontracted the fabrication rights of these chips to AMD, Harris, Hitachi, IBM, Siemens, and possibly others. Amongst Intel and their cohorts, the 8086 line of processors ran at speeds ranging from 4 MHz to 16 MHz.

It didn’t take long for the industry to start "cloning" the IBM-PC. Many companies tried; but mostly they all failed because their BIOS was not compatible with the IBM-PC BIOS. Columbia, Kayro and others went by the wayside because they were not totally PC-compatible. Compaq broke though the compatibility barrier with the introduction of the Compaq portable computer. Compaq's success created the turning point that enabled today's modern computer industry.

NEC was the first to "clone" this new Intel chip with their V20 and V30 designs. The V20 was pin-compatible with the 8088, while the V30 was pin-compatible with the 8086. The V-series ran approximately 20% faster than the Intel chips when running at the same clock speed. Therefore, the V-series chips provided a cheap "upgrade" to owners of the IBM-PC and other clones computers. The V-series chips were very interesting. These chips were introduced in 1985 at approximately the same time as Intel's introduction of the 80386. The 80386 was still years away from production, and the 80286 was just barely being accepted in the IBM-PC/AT. Even though these chips were pin-compatible with the 8086 and 8088, they also had some extensions to the architecture. They featured all of the "new" instructions on the 80186 / 80188, and also were capable of running in Z-80 mode (directly running programs written for the Z-80 microprocessor).



Glossary of PC Processors by x86.org
CISC Complex Instruction Set Computer. A CISC microprocessor is one in which the number of bytes needed to represent the opcode instruction is not a fixed, regular length (for example, 32-bits each). (See also RISC, EPIC, and VLIW.)
Opcode The data that represents a microprocessor instruction.
RISC Reduced Instruction Set Computer. A RISC microprocessor that has fewer, simpler instructions than its CISC counterpart. RISC instructions perform simple, rudimentary functions. The simplicity of these instructions results in a very simple microprocessor design that can execute very fast. RISC instructions are typically characterized by fix length instruction sets (for example, all instructions are 32-bits each). (See also CISC, EPIC, and VLIW.)
EPIC Explicitely Parallel Instruction Computing. EPIC is a fancy acronym that Intel invented to obfuscate the fact that they do not want the public appearance that their Merced microprocessor is actually a VLIW design. After all, Intel didn't invent VLIW, therefore they don't want to be publicly associated with a VLIW design.
VLIW Very Long Instruction Word. A microprocessor that packs many simple RISC-like instructions into a much longer internal instruction word format. A VLIW microprocessor will usually have execution units, capable of executing all of the instructions contained in the instruction word, in parallel.
MMX Multi-Media eXtensions. Intel claims that "MMX" is not an acronym, meaning "Multi-Media eXtensions" because they have filed for a trademark under this name. In reality, MMX instructions are intended to enhance programs that have multi-media capabilities.
Address Lines The number of address lines, or "address pins" on a microprocessor determine how much memory the chip can address. The amount of addressible memory can be calculated as 2^#address_lines (two raised to the power of the number of address lines). A microprocessor with 32 address lines can address 232 bytes of memory (4 G Bytes).
Cache A cache is a bank of high speed memory that stores the most recently accessed code and data. When the microprocessor requests data that is in the cache, the amount of time to retrieve the data is many times less than the amount of time needed to access main memory. Many microprocessors have a cache inside of the chip itself. In some cases, there is a cache for the cache (known as a 2nd-level cache). A cache may hold code, data, or even recently accessed data on a hard disk. In general, a cache can be created for faster access to any slower device, beit main memory or hard disks.

Copyright 1998, Intel Secrets Web Site and Robert R. Collins